1. Field of the Invention
The present invention relates to apparatus and methods of breaking fracturing fluids.
More particularly, the present invention relates to apparatus and methods of breaking fracturing fluids, where the method includes that step of supplying a composition to a fracturing fluid prior to or after initial fracturing, where the composition includes a microbial agent capable of digesting or converting a gelled component of the fracturing fluid into low molecular components resulting in a “breaking” of the viscosity of the fracturing fluid over a set and controllable period of time and to compositions and methods for making the compositions.
2. Description of the Related Art
Most wells are hydraulically fractured to increase flow. The rheological requirements of a fracturing fluid are highly constraining. To adequately propagate fractures in the subterranean formation, the fracturing fluid is designed to have properties such as body, viscosity, etc. sufficient to form fractures in the formation without leaking excessively into the formation, when the fracturing fluid is forced into the formation at elevated pressures. Also, a fracturing fluid is designed to have the capability to transport and deposit large volumes of proppant into the fractures or cracks in the formation formed during fracturing. After the fracturing operation is complete and pressure on the fluid is released, the fracturing fluid is designed to readily flow back into the well and not leave significant residues in the fractures that impair permeability of the formation and conductivity of fluids into and out of the fractures. Finally, a fracturing fluid is designed to have rheological characteristics which permit it to be formulated on the surface with reasonable convenience and to be pumped down the well without excessive difficulty or pressure drop frictional losses.
The most commonly used fracturing fluids are water-based compositions containing a water soluble hydratable high molecular weight polymer, which increases the viscosity of the fluid by forming a gel when it dissolves in the fluid. Thickening the fluid reduces leakage of liquids from the fracturing fluid into the formation during fracturing and increases proppant suspension capability.
In some fracturing fluid formulations, chemical agents are added to crosslink the polymer viscosifier molecules to further increase fluid viscosity. Cross-linking increases fluid viscosity by forming interpolymer chemical bonds.
When the fracturing operation is complete, the pressure of the fracturing fluid in the formation is reduced. Fracturing fluid flows back out of the formation into the well. Fracturing fluids are designed to flow quickly and completely out of the formation and back into the well to allow production of hydrocarbons. Although hydratable polymers naturally decompose over time, these natural degradation processes are generally too slow resulting in too great of a loss of production time if producers were required to wait for natural degradation processes to break the fracturing fluid viscosity. To enhance back flow of fracturing fluid out of the formation and into the well, compounds are added to the fluid (initially or subsequent to fracturing) to reduce or “break” the viscosity of the fracturing fluid so that the fluid can flow more freely and be removed from the formation into the well more quickly.
Fracturing fluid viscosity breaking also is utilized to minimize damage to the formation. As the fracturing operation proceeds, the thickening agents in the fracturing fluid can form a thin film over the fracture face which is referred to as a “filter-cake.” Excessive filter cakes can imped the flow of production fluids from the formation into the well.
It has been reported in the literature that enzymes can be used to degrade drilling fluid residues. For example, Hanssen, et al., “New Enzyme Process for Downhole Cleanup of Reservoir Drilling Filter cake” SPE 50709 (1999) disclosed experimental work towards the use of enzymes for downhole cleanup of filter cakes produced by water-based drilling fluids.
U.S. Pat. No. 5,247,995 (incorporated herein by reference) disclosed method of degrading damaging material within a subterranean formation of a well bore using an enzyme treatment. Filter cakes and very viscous fluids are such damaging materials. The enzyme treatment degrades polysaccharide-containing filter cakes and damaging fluids which reduces their viscosity. The degraded filter cake and damaging fluid can then be removed from the formation back to the well surface. The particular enzymes utilized are specific to a particular type of polysaccharide and are active at low to moderate temperatures. The enzymes attack only specific linkages in filter cakes and damaging fluids and are active in the pH range of about 2.0 to 10.0.
Simply adding the enzymes to the fluid may lead to premature breaking of the fluid. U.S. Pat. No. 5,437,331 disclosed a method of fracturing a subterranean formation in a well bore is shown in which a gellable fracturing fluid is first formed by blending together an aqueous fluid, a hydratable polymer, a suitable cross-linking agent for cross-linking the hydratable polymer to form a polymer gel and an encapsulated enzyme breaker. The cross-linked polymer gel is pumped into the well bore under sufficient pressure to fracture the surrounding formation. The encapsulated enzyme breaker is allowed to degrade the cross-linked polymer with time to reduce the viscosity of the fluid so that the fluid can be pumped from the formation back to the well surface. The particular enzyme breaker uses open cellular encapsulation to protect and delay the action of the enzyme.
Although enzyme type breakers have been used, there use has been limited do to release dynamics and duration of activity, there is still a need in the art for compositions and methods of breaking a fracturing fluid in a controlled manner over a controlled or designed period of time.